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| -rw-r--r-- | compiler/utils/Maybes.lhs | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/compiler/utils/Maybes.lhs b/compiler/utils/Maybes.lhs index 1d43365dce..d4f8346f14 100644 --- a/compiler/utils/Maybes.lhs +++ b/compiler/utils/Maybes.lhs @@ -17,6 +17,7 @@ module Maybes ( expectJust, maybeToBool, + MaybeT(..) ) where import Data.Maybe @@ -82,6 +83,26 @@ orElse :: Maybe a -> a -> a Nothing `orElse` y = y \end{code} +%************************************************************************ +%* * +\subsection[MaybeT type]{The @MaybeT@ monad transformer} +%* * +%************************************************************************ + +\begin{code} + +newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} + +instance Functor m => Functor (MaybeT m) where + fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x + +instance Monad m => Monad (MaybeT m) where + return = MaybeT . return . Just + x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f) + fail _ = MaybeT $ return Nothing + +\end{code} + %************************************************************************ %* * |